Laurence L. Henry

Synthesis of poly(methyl methacrylate) stabilized colloidal zero-valence metallic nanoparticles

Poly(methyl methacrylate) (PMMA) stabilized colloidal metallic iron nanoparticles without additional surfactant or stabilizer were fabricated by a wet chemical reduction method. The synthesized colloidal iron nanoparticles were found to be stable in tetrahydrofuran (THF) solution and the dried PMMA–Fe nanocomposites were ferromagnetic even at room temperature as illustrated by magnetometer measurement. The particle size increased with the decrease of the initial PMMA concentration and the magnetic properties also depended on the initial PMMA concentration.

Stable Amorphous Cobalt Nanoparticles Formed by an in Situ Rapidly Cooling Microfluidic Process

The controlled synthesis of nanoparticles (NPs) with stable crystal structures and stable physical and chemical properties is a key issue for commercial applications. The use of a microfluidic reactor (MR) process has proven to be a flexible approach to control the fine crystal structures and the magnetic properties during the ripening and aging of the NPs. We have developed an in situ rapidly cooling microfluidic process (IRCMP) in which Co NPs with stable crystal structures and magnetic properties are synthesized by using elevated reaction temperatures followed by rapid quenching of the colloids to reduced temperatures. The Co NPs that are obtained by this process demonstrate stable crystal structures and stable magnetic properties for a much longer period of time (at least 3 months) than for Co NPs obtained by performing the reaction and the quenching processes at room temperature or under sonication.

Specific heat of aligned multiwalled carbon nanotubes

The specific heat of an aligned bulk multiwalled carbon nanotube sample made by the chemical vapour deposition method was measured from 250 to 1.8 K. The specific heat curve gradually decreased, showing one-dimensional (1D) behaviour down to 40 K. Below 40 K it showed a rapid decrease due to the dimensional change from 1D to 3D behaviour, indicated by different temperature dependences at low temperature. Interestingly, a T−2 term was observed below 5 K, suggesting a nuclear hyperfine component due to magnetic impurities which were confirmed to be present by thermogravimetric analysis and microscopy observations.

Displacement Synthesis of Cu Shells Surrounding Co Nanoparticles

Copper shells were fabricated by a displacement method around Co nanoparticles (3.2 ′ 0.6 nm) at room temperature in a copper-citrate aqueous electrolyte. The nanoparticles were synthesized by a wet chemical approach using the surfactant sulfobetaine, dodecyldimethyl (3-sulfopropyl) ammonium hydroxide (98%) in tetrahydrofuran. X-ray absorption near-edge structure analysis confirmed that cobalt oxide was not present in the nanoparticles upon exposure to air, consistent with a shell formation. Additionally, the presence of the shell resulted in an increase of the blocking temperature of the core-shell nanoparticles, stabilizing the ferromagnetic behavior up to 235 K.

Magnetoresistance and Annealing Behaviors of Particulate Co–Au Nanocomposites

Co core Au shell nanoparticles, stabilized with a sulfobetaine surfactant, were fabricated from a displacement reaction of Au 3+ with Co nanoparticles and compressed into a granular composite. The affect of annealing the composite in a hydrogen environment was investigated and found to have a dramatic effect on the magnetic properties, size, and composition of the CoAu nanoparticles. A negative magnetoresistance was observed and exhibited a parabolic functionality with annealing temperature, increasing and then decreasing with annealing temperature. A similar behavior was observed for the coercivity, attributed to the increase in particle size with the annealing temperature. The surfactant was decomposed with annealing.

Nearly Monodispersion CoSm Alloy Nanoparticles Formed by an In-situ Rapid Cooling and Passivating Microfluidic Process.

An in siturapid cooling and passivating microfluidic process has been developed for the synthesis of nearly monodispersed cobalt samarium nanoparticles (NPs) with tunable crystal structures and surface properties. This process involves promoting the nucleation and growth of NPs at an elevated temperature and rapidly quenching the NP colloids in a solution containing a passivating reagent at a reduced temperature. We have shown that Cobalt samarium NPs having amorphous crystal structures and a thin passivating layer can be synthesized with uniform nonspherical shapes and size of about 4.8 nm. The amorphous CoSm NPs in our study have blocking temperature near 40 K and average coercivity of 225 Oe at 10 K. The NPs also exhibit high anisotropic magnetic properties with a wasp-waist hysteresis loop and a bias shift of coercivity due to the shape anisotropy and the exchange coupling between the core and the thin oxidized surface layer.

Discovery of the Griffiths phase in the itinerant magnetic semiconductor Fe1-xCoxS2.

Critical points that can be suppressed to zero temperature are interesting because quantum fluctuations have been shown to dramatically alter electron gas properties. Here, the metal formed by Co doping the paramagnetic insulator FeS2, Fe1-xCoxS2 is demonstrated to order ferromagnetically at x > xc = 0.01+/-0.005, where we observe unusual transport, magnetic, and thermodynamic properties. We show that this magnetic semiconductor undergoes a percolative magnetic transition with distinct similarities to the Griffiths phase, including singular behavior at xc and zero temperature.

A new role for surfactants in the formation of cobalt nanoparticles

During wet-chemical synthesis of nanoparticles it is believed that surfactants interact with the particles after their nucleation thereby controlling their crystal structure, shape and size. Contrary to this, our investigations presented in this paper reveal a new role for surfactants: influencing the reaction pathways, prior to nucleation, leading to the formation of cobalt nanoparticles. The results from the mechanistic investigation of the influence of various surfactants on the formation of cobalt nanoparticles carried out using time dependent FT-IR spectroscopy support this observation. The utilization of different surfactants such as oleic acid (OA), trioctylphosphine oxide (TOPO), octadecylamine (ODA), and trioctylphosphine (TOP) led to differences in reaction pathways and reaction intermediates, prior to the nucleation, leading to the formation of Co nanoparticles with very different properties. The particle size and size distribution were obtained from transmission electron microscopy (TEM). The electronic and geometric properties of the cobalt nanoparticles obtained were determined using synchrotron radiation based X-ray absorption spectroscopy (XAS) and the magnetic properties were measured using SQUID magnetometry.

Magnetic and thermodynamic properties of cobalt-doped iron pyrite: Griffiths phase in a magnetic semiconductor

Doping of the band insulator FeS

Development of a Smart Material Integrated Sensor Thermal Switch for Thermal Management

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